A lizard, especially a small one, entering your house is not a real problem. Lizards don't normally chew wiring, bite people or spread disease. In some places, small lizards such as geckoes regularly come inside and can be quite useful, because they eat nuisance insects such as mosquitoes. Not everybody wants a lizard in their living room though. If the lizard is not welcome, catch it and put it back into its natural habitat.
Close doors leading to the room the lizard is in. This prevents the lizard scampering off through the rest of the house.
Open the windows. The lizard might find its own way out while you are trying to catch it.
Approach the lizard slowly, not letting your shadow fall on it, and place the aquarium net over it quickly. If you don't have a net, use an empty plastic tub instead.
Slide the cardboard underneath, hold the cardboard and net or tub together and take the lizard outside.
If you regularly find lizards inside, check that they are not coming inside to feed on bugs. A cockroach infestation, for example, will attract lizards and needs to be dealt with.
Never try and grab a lizard with your hands. If you grab the tail, the tail will come off. The lizard won't die but this does it no good at all and will give you a shock. If you grab its body, you might fatally damage it. Aside from the animal welfare angle, some lizards are endangered species. Further, reptiles sometimes carry salmonella. If you do touch the lizard, wash your hands thoroughly afterwards.